The present invention relates to a position detecting apparatus for use with a magnetic scale and a rotary encoder applicable to machine tools, industrial machines, precision type measuring/angle measuring apparatus or the like, for example.
Magnetoresistance effect elements (often referred to as "MR elements") made of a thin film such as Fe--Ni (permalloy) or Ni--Co exhibiting a magnetoresistance effect are generally used as a magnetic detecting portion of a position detecting apparatus, such as a magnetic scale or rotary encoder applicable to machine tools, industrial machines and precision type measuring/angle measuring apparatus.
Shapes and structures of magnetic scales and rotary encoders with magnetic detecting portions composed of MR elements are changed variously depending on the purpose and manner in which they are used. Fundamentally, magnetic scales and rotary encoders with high accuracy and high resolution can realize high precision and high resolution by reducing the magnetic recording pitch (recording wavelength) or reducing the error produced per wavelength when they are inserted into position detecting apparatus by reducing a harmonic distortion of a waveform or improving a signal-to-noise ratio (S/N).
The above magnetic detecting portion generally comprises thin film elements formed on a glass or silicon base. Magnetic scales and rotary encoders with high precision and high resolution using such a magnetic detecting portion hold a magnetic recording scale surface and a MR element surface serving as a detecting surface at a constant spacing parallel to each other such that the two surfaces can be relatively moved in an opposing relation in order to reduce the recording wavelength.
A waveform of an output from the magnetic detecting portion contains a harmonic distortion due to mainly a saturation characteristic of the magnetoresistance effect in the MR element. A pattern layout of the MR element is closely studied in order to lessen the harmonic distortion. In general, the MR element has a pattern layout of a so-called spatial filter type in order to reduce a specific harmonic distortion component.
The position detecting apparatus such as the magnetic scale and rotary encoder can detect a moving amount and a moving direction based on two sine wave signals having a phase difference of 90 degrees output from the magnetic detecting portion. The position detecting apparatus arranged such that the magnetic recording surface and the detection surface of the magnetic detecting portion are relatively moved with a constant spacing therebetween in parallel to each other encounters the following problems:
Specifically, when a clearance between the scale surface and the detection surface of the magnetic detecting portion or spacing between the scale surface and a magnetic detecting portion holding mechanism is smudged by foreign substances such as dusts or chips or the original scale surface is curved or wobbles, such clearance or spacing is widened or relative angles (pitch, yaw, azimuth) are fluctuated. Therefore, levels of output signals or DC components are fluctuated and the detection accuracy of the position detecting apparatus is often degraded.
When a magnetic scale has a fluctuation of a magnetic characteristic of a magnetic material forming a magnetic recording medium constructing the magnetic scale or a deviation of a material, output signals are similarly fluctuated and the detection accuracy becomes degraded.
When the relative angle is fluctuated, the magnetic characteristics of the magnetic material are fluctuated or when the material is not well-balanced, amplitudes of the two sine wave signals with a phase difference of 90 degrees are separately changed in the position detecting apparatus using MR sensors, which mainly causes the detection accuracy to be degraded.
When the relative angles (pitch, yaw, azimuth) between the surface of the magnetic recording medium and the detecting surface of the magnetic detecting portion is fluctuated, effects achieved by the pattern layout reducing the harmonic distortion is deteriorated, which leads to the deterioration of the detection accuracy.
Furthermore, the magnetoresistance effect element is the resistor and therefore generates heat by application of a current so that the temperature of the magnetoresistance effect element increases. Increasing temperatures of the respective magnetoresistance effect elements are changed depending on the pattern layouts. As a result, the resistance values and the changing ratios of the magnetoresistance effect elements are fluctuated depending on the temperature characteristics of the magnetoresistance effect elements to cause a drift of an output voltage and a fluctuation of amplitude, resulting in the detection accuracy being degraded.